The present invention relates to permanent magnet synchronous machines and more particularly to a method and apparatus for determining the position of a motor rotor.
To assure maximum torque per amp output in a permanent magnet synchronous motor, the control must know the rotor position or angle with respect to the stator. The torque constant is proportional to the cosine of electrical angle misalignment, and an offset of only 10 degrees translates to about 1.5 percent reduction in maximum torque per amp.
In prior art devices, high accuracy absolute feedback devices have been used. These devices are pre-aligned, and store an offset angle to the magnetic axis for the drive to read at power-up. Other types of permanent magnet synchronous motors include devices such as resolvers that are physically aligned and clamped, usually with the aid of a custom test fixture supplied by the drive manufacturer. While these integrated motor/feedback methods are commonly used in motors for controlling motion, they are not used with standard or general purpose drives because in these applications, the motor and feedback are treated separately. Even absolute devices used in standard drives must therefore undergo initial alignment to determine offset angle.
Other types of encoder devices can also be used. In particular, incremental encoders are used to provide relatively inexpensive rotor feedback. These devices, however, lack absolute position and also must be re-aligned whenever power is interrupted.
Therefore, while a number of methods are known for determining the rotor position in a permanent magnet synchronous motor, these devices all add significant cost to the device. It is becoming increasingly important, however, to reduce the cost of permanent magnet synchronous motors. The following outlines a method for determining the rotor position that is inexpensive, simple, accurate and relatively maintenance free.